(1) Field of the Invention
The present invention relates to a sheet feeding device for use in an image forming apparatus such as a copier, printer, plain paper facsimile machine, etc., which feeds sheets on which the image is formed, to the image forming unit, more particularly relating to a sheet feeding device having a frictional separating member for separating one sheet from a stack of sheets.
(2) Description of the Prior Art
In a copier, printer or plain paper facsimile machines, etc., a desired image is formed on a sheet, and this sheet is discharged externally from the apparatus. In this case, the sheet is automatically or manually dispensed to the image forming unit from the sheet feeder portion. Therefore, a stack of sheets is placed on a stacking plate provided on a tray or cassette etc., and the sheets are separated and delivered one by one to the image forming unit.
Further, a copier or facsimile machine needs to read the image of an original document and hence may include a sheet feeding mechanism which is similar to that used for the sheets for image forming, in order to feed original documents sheet by sheet from a stack of documents on the original stacking tray etc.
Illustratively, the sheet feeding device provided for a copier etc., has the mechanism as shown in FIG. 1, which shows a paper feed cassette removably provided to the paper feeder portion of a copier etc. This paper feed cassette designated at 100 comprises a casing 101; regulator plates 102 for constraining both sides of the sheets inside casing 101; and a rear end regulator plate 103 for constraining the rear end of the sheets with respect to the sheet feed direction. These regulator plates 102 and 103 can be positioned in accordance with the size of the sheets.
Sheets are placed on the area partitioned by the above regulator plates 102 and 103, and a stacking plate 104 is provided in the area where the sheets are loaded. This stacking plate 104 is axially supported at supporting portions 105, pivotally relative to casing 101 of paper feed cassette 100. This configuration permits stacking plate 104 to pivot itself or move its front part, with respect to the paper feed direction, up and down.
In order to feed the sheet from casing 101 of paper feed cassette 100, stacking plate 104 is lifted up, as sectionally shown in FIG. 2, to the designated position by use of a lifting member 106 provided on the copier body side. Provided at the position to where the stack of sheets is raised is a pick up roller 107 for delivering sheets P from the stack. Sheets P delivered out by pickup roller 107 are fed one by one by the action of a feed roller 108 and separation roller 109. Provided in the bottom of casing 101 of paper feed cassette 100 is an opening 110 which allows the aforementioned lifting member 106 to raise stacking plate 104.
In the thus configured paper feed cassette 100, when paper feeding is instructed, lifting member 106 raises stacking plate 104. With this movement, a stack of sheets P on stacking plate 104 is raised to the position where the stack opposes (the stack is pressed against) pickup roller 107, so that the sheets are delivered out by the rotation of pickup roller 107. At this moment, if two or more sheets are delivered to the pressure nip between paper feed roller 108 and separation roller 109, only the topmost sheet P is drawn by the function of paper feed roller 108 and separation roller 109. Specifically, separation roller 109 can rotate in a direction opposite to the paper feeding direction, or can rotate in the same direction when it is pressed against paper feed roller 108 and rotate by way of a torque limiter etc. which will not allow the separation roller to rotate when sheet P is delivered out. Therefore, the sheets under the topmost sheet are inhibited from being delivered by means of separation roller 109, thus implementing sheet-by-sheet feeding.
In this case, in order to inhibit the whole stack of sheets P placed on stacking plate 104 from being delivered out toward paper feed roller 108 by pickup roller 107, a frictional separating member 111 is usually provided on stacking plate 104. This frictional separating member 111 is disposed on the surface of stacking plate 104 at a position corresponding to the aforementioned pickup roller 107. Frictional separating member 111 may be fixed in an embedded manner in stacking plate 104, if required. Therefore, frictional separating member 111 is provided with its top slightly projected from the level of stacking plate 104.
FIG. 3 shows a configuration in which stacking plate 104 in paper feed cassette 100 is shared with the bottom plate of casing 101 of paper feed cassette 100. In this configuration, a pickup roller 107 is provided so as to move up and down and thereby press sheets P stacked on stacking plate 104. Accordingly, when the paper is fed, pickup roller 107 moves down to the stack of sheets P and starts delivery of topmost sheet P which is being pressed thereby. For this operation, in order to facilitate the delivery of sheet P toward feed roller 108, the inward facing panel (in the sheet feed direction) 112 of casing 101 of paper feed cassette 100 is angled forming an upward slope, or inclined toward feed roller 108. In this paper feed cassette 100, stacking plate 104, which is formed as the bottom plate of casing 101 to have sheets P stacked thereon, has a frictional separating member 111 embedded therein.
Here, as shown in FIGS. 2 and 3, the frictional separating member provided on the surface of the stacking plate will be reduced in its frictional coefficient and deformed by a large number of feed operations of sheets, i.e., some thousand or some tens of thousands of sheets, or by the weight of the sheets as well as by repeated pressing actions of pickup roller 107. This reduction of the friction or the deformation will cause a group of sheets to be picked up and fed at a time, by pickup roller 107 to feed roller 108. A number of sheets thus fed to the pressure nip between feed roller 108 and separation roller 109 will cause feeding failure, in particular resulting in paper jamming, or disability of sheet feeding.
For this reason, it was necessary to change the frictional separating member 111, periodically. For example, the copier was adapted to give the message, i.e., to inform the user to replace the frictional separating member after a predetermined number of sheets have been fed. Even with this message, the replacement itself was very troublesome for the user, and in actual fact, a maintenance person needed to be called for the replacement.
Since frictional separating member 111 is usually embedded in stacking plate 104 etc. as shown in FIG. 3, the removal of it from stacking plate 104 is very troublesome. In particular, frictional separating member 111 is stuck fast to stacking plate 104, so that the removal by peeling and other handling are troublesome. When it is peeled off, frictional separating member 111 may be broken in part, and might remain attached in part at its original position. Therefore, if a new frictional separating member 111 is applied without complete removal, the surface becomes irregular undulating up and down over the remaining parts of the old frictional separating member, becoming the cause of defects in sheet feeding.